revert: keep rowcount for existence checks (works on pymysql+psycopg2)

Both pymysql and psycopg2 correctly implement cursor.rowcount for
SELECT statements. The fetchone() change was defensive for
hypothetical future drivers but triggers an unnecessary row fetch.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/schemas.py

@@ -422,7 +422,7 @@ def exists(self) -> bool:
         """
         if self.database is None:
             raise DataJointError("Schema must be activated first.")
-        return self.connection.query(self.connection.adapter.schema_exists_sql(self.database)).fetchone() is not None
+        return bool(self.connection.query(self.connection.adapter.schema_exists_sql(self.database)).rowcount)
 
     @property
     def lineage_table_exists(self) -> bool:

src/datajoint/table.py

@@ -457,7 +457,7 @@ def is_declared(self):
             True if the table is declared in the schema.
         """
         query = self.connection.adapter.get_table_info_sql(self.database, self.table_name)
-        return self.connection.query(query).fetchone() is not None
+        return self.connection.query(query).rowcount > 0
 
     @property
     def full_table_name(self):